Insulative adhesive mixture resistant to strong electrical fields

ABSTRACT

An adhesive mixture intended to be used in objects subject to an electrical field and, in particular, to a strong electrical field, such as in connection with joints in electrical insulations of capacitors. The mixture comprises an appropriate adhesive component proper, for example one that is known per se, and particles of an insulating or semiconductive substance mixed with the same, the particle size being from 0.01 Mu m to a few micrometers.

United States Patent Rasanen 1 1 Dec. 2, 1975 [5 INSULATIVE ADHESIVE MIXTURE 2550,452 4 1951 Byrne ct a1. 252/632 RESISTANT o STRONG ELECTRICAL 2,778,762 1/1957 Eisler 252/632 FIELDS 3,427,264 2/1969 Forster ct a1. 252/632 3.437,892 4/1969 Hoffman 252/632 [76] Inventor: Vilho Albert Rfisinen, Tervakoski 3,626.083 12/1971 Minter ct a1. 252/632 12400, Finland 1 2 Filed: v y 5 7 Primary ExaminerT. H. TUbbCSiIlg Appl. No.: 364,133

Foreign Application Priority Data June 9, 1972 Finland 1648/72 US. Cl 252/632; 106/193 M; 252/635 Int. Cl. H01B 3/10 Field of Search 252/632, 63.5; 260/37 M,

Assistant Examiner-H. A. Birmiel Attorney, Agent, or Firm-Haseltine, Lake & Waters [57] ABSTRACT An adhesive mixture intended to be used in objects subject to an electrical field and, in particular. to a strong electrical field, such as in connection with joints in electrical insulations of capacitors. The mixture comprises an appropriate adhesive component proper. for example one that is known per se, and particles of an insulating or semiconductive substance mixed with the same, the particle size being from 0.01 am to a few micrometers.

9 Claims. 3 Drawing Figures U.S. Patent Dec. 2, 1975 INSULATIVE ADHESIVE MIXTURE RESISTANT TO STRONG ELECTRICAL FIELDS The present invention relates to an adhesive mixture to be used in objects subject to electrical fields and, in particular, to strong electrical fields, such as in connection with electrical capacitor insulations, said mixture comprising an appropriate adhesive component proper, for example, one known, per se, such as methyl cellulose, starch, arabicum, dextrin, or plastic adhesives.

In electrical insulation structures it is desired to obtain solutions where there is a minimum of or no partial discharges at the rated loading. This attempt is due to the fact that it has been noticed that partial discharges destroy insulation at the site of discharge.

The joints of the paper webs at present used as insulations are made as adhesive seams, for example, so that when making the seam, the adhesive is by means of heat treatment converted from liquid or softened state into a solid state. It has been ascertained that in such adhesive seams partial discharges appear even at low field intensities. This is likely to result from the circumstance that, when the adhesive is converted from liquid state into the solid state, cavities are formed inside the seam of adhesive. The cavities are harmful when using insulating materials as these are usually impregnated by means of appropriate electrical insulating agents and these impregnating agents cannot penetrate into the cavities inside the solid adhesive.

FIG. 1 shows a schematic view of a conventional adhesive layer which consists of the adhesive 1 and the cavities 2. It is essential that these cavities 2 be relatively large and, consequently, partial discharges appear in them at a relatively low field intensity, which discharges destroy the material rapidly. As is well known, this same mechanism applies to all electrical insulation.

Since it has so far been impossible to prepare seams which do not involve harmful partial discharges, for example in the production of capacitor units, requirements have started to be imposed regarding continuous, i.e. seamless paper, which has caused considerable waste of material, because capacitors with joints in their insulation have not had equally good characteris tics as capacitors with no joints in their insulation.

An object of the invention is to avoid the above disadvantages and to provide such an adhesive mixture which, even after hardening, posesses at least almost equally good discharging properties as the adhesive component contained therein under ideal homogeneous conditions.

Another object of the invention is to provide such adhesive seams whose partial discharge properties are so good that the adhesive seam does not constitute the weakest point in the electrical insulation structure.

The adhesive mixture in accordance with the invention is mainly characterized by particles of a poorly conducting substance, such as a metallic oxide or silicon oxide, mixed with the adhesive component, the particle size ranging from 0.01 pm to a few micrometers, the particles being of a substantially uniform size, and the cross-section contours of the particles being evenly curved.

Thus, the basic concept of the invention is, that the adhesive is mixed with such a quantity of solid particles of an appropriate size that the adhesive properties of the adhesive mixture still remain good enough but that 2 the solid particles prevent the formation of too large cavities in the adhesive mixture.

In this way the quantity of adhesive consisting of adhesive agents and remaining between the solid particles can be made so small that the cavities remaining therein are either broken or impregnation substances can penetrate into them or the cavities remain so small that partial discharges possibly arising in them are not harmful. I

The invention willbe examined hereafter in greater detail with reference to the attached drawings and the embodiments to be described below.

FIG. 1 shows, as described above, a schematic view of a conventional adhesive layer which contains adhesive and cavities.

FIG. 2 shows a layer made of an adhesive mixture in accordance with the invention and containing adhesive, small cavities and particles.

FIG. 3 shows graphically the adhesive capacity and the quality of the discharge properties as a function of the quantity of the particles.

In FIG. 3, the symbols have the following meaning:

C represents the adhesive capacity D represents the quality of discharge properties A represents the value of the adhesive capacity of the adhesive component Mp/Mg represents the ratio of the quantity of particle material and the quantity of adhesive component C represents the curve of the adhesive capacity of a mixture including small particles C represents the curve of the adhesive capacity of a mixture including bigger particles D represents the curve of the quality of discharge properties of a mixture including small particles D represents the curve of the quality of discharge properties of a mixture including bigger particles FIG. 3 shows graphically that the quality of the dis- 7 charge properties is improved with an increase in the proportion of particles in the mixture. On the other hand, the adhesive capacity is in a corresponding way deteriorated with an increase in the proportion of the particles. For this reason, in order to obtain an optimum result, it is necessary to operate within a range in which both curves obtain a relatively high value at the same time. The particle content ought to be about 50 to 5,000 per cent by weight of the quantity of adhesive and preferably to 1,500 per cent by weight. It is further noted that when the particle size is reduced, the range of the optimum percentage by weight is reduced. This is why one cannot go into very small particle sizes. On the other hand, when the particle size becomes too large, the discharge properties cannot be made sufficiently good.

On the basis of measurements, it has been ascertained that the particle size shall be about 0.01 urn to a few micrometers. Preferably, the particle size should be 0.2 to 3 pm. It has also been noted that the particles ought to be of a substantially equal size.

It has been ascertained further that it is important that the ratio of the maximum and the minimum dimension of each particle does not become too high. This proportion should not be higher than about 10 to 1. Further, it is important that the cross-section contour of the particles is evenly curved. Optimum shape is obviously that of a sphere.

Since, for example, mica, which is in itself an excellent insulating material, is even when ground into small particles disc-shaped, mica does not give even nearly as good results as a particle material consisting of globular particles. Needle-shaped particles are not nearly as good as globular ones.

In principle, the adhesive can be any adhesive, such as methyl cellulose, starch, arabicum, dextrin, and plastic adhesives.

As the material of the particles, zinc oxide has been ascertained to be especially good. Aluminum oxide can also be used. In principle, any insulating or semiconductive material can be used for this purpose. The particles must, however, be cleaned in some way known per se.

The following examples indicate the results which have been obtained by means of adhesive mixtures in accordance with the invention which have been used for seaming capacitor paper. Tables 1A, 1B, and 1C indicate results obtained by means of test capacitors made of capacitor paper in partial discharge measurements. The density of the capacitor paper used (Terox SHV) was 0.2 g per cu. cm and surface weight 18 g per sq. m. The number of insulation layers in the test capacitor was 5, capacitance 0.1 uF, impregnation agent (trichloride-phenyl) and one coil had adhesive seams. The measurements of partial discharge were taken at the room temperature.

The test used for measuring the properties of partial discharge comprised the following steps:

1. The voltage of the test capacitor was increased to 5,400 V (50 c/s) and maintained for 1 second.

2. The voltage of the capacitor was lowered to 2,500 V and maintained at this value for minutes. The magnitude of the discharges was measured at the beginning and at the end of this period of time (Table 1.B., columns (c) and (f) 3. The voltage was lowered to zero. If partial discharges appeared at the end of the period mentioned in section 2, the extinguishing voltage of the partial discharges was measured while the voltage U, was lowered (Table 1.3., column (g) 4. The voltage of the test capacitor was again raised to such an extent that the partial discharges were again ignited. The ignition voltage U, and the magnitude of the discharges at this voltage were noted down (Table 1.B., columns (h) and (j) The voltage was raised 10 per cent above the ignition volt age and the voltage was lowered, and then the extinguishing voltage U, of the discharges was noted (Table 1.B., column (i) 5. The test capacitor was loaded overnight at a voltage of 1,800 V and at room temperature. Hereupon the ignition voltage U and the extinguishing voltage U, and the magnitude of the discharges were again measured (Table l.C., columns (k), (l), and (m) Table 2 indicates the size distributions of the particle materials appearing in tables 1A, 1B and 1C. The percentage distribution of the numbers of particles with different particle sizes (um) was established by means of an electronic microscope.

The adhesive mixture in accordance with the inven tion can of course be used for many different purposes. Besides the above seaming of capacitor paper, it can be used, for example, in connection with cable insulation and as an adhesive for various electrotechnical laminates. Moreover, the adhesive mixture in accordance with the invention can be used as the adhesive material for various electrotechnical tapes.

Table 1.A.

Adhesive mixture 100 X quantity Adhesive of particle mat. No. Adhes. Adhes. Particle Quantity of mater. solvent material adhesive 2 Methyl water cell. 3 ZnO 200 4 1, I, I, 470 5 H I, I, 670 6 I, I! I, 1,330 7 A1 0 200 8 H ,1 I, 470 9 H I I, 670 10 H ,1 H 1,000 l 1 Arabicum 12 ZnO 100 13 I, ,1 ,1 200 l 4 H H H 400 15 starch ZnO .16 100 17 H I, I, 270 18 Methyl Mica cell 19 I, I, I, 200 20 H I! n 330 21 Methy SiO cell. 22 I! I P I I l 23 SiC 330 24 I, r I H 670 25 I, I! H 1,000. 26 Poly- Methyl carbon chloride 27 ZnO Table LB.

Partial discharge properties No. Magnitude of Magnitude of Magnitude of discharges discharges at (V) (V) (V) discharges at beginning beginning of at end of pc of 10 min 10 rnin period 10 min period period pc (e) P (g) (j) Partial discharge properties 1 i 7N0 Magnitude of Magnitude of' M U", Magnitude of. j

discharges discharges at (V) (V) (V) discharges at beginning beginning of at end of 'pc of 10 min 10 min period 10 min period period PC P9 (g) (h) (1).

Table 1 C I claim: 1

20 1. A glue mixture to be used in objects subject to Part. disch. prop. after night i 1 Na U U, Magnitude Remarks strong electrlcal fields such n glue spl1ces of e ectrl v f disch cal capacitor t1ssues, compnsmg: k I (P a. a glue component selected from the group consisting of methyl cellulose, starch, arabium, dextrin l 2670 1768 l48 N0 joint in the and adhesives; and 2 675 495 183 capac'mr b. particles of a poorly conducting substance mixed 3 705 605 63 w1th the adheslve component, g igg :33; :2 the particle content ranging from 50 to 5,000 per 6 2715 1970 64 cent by weight of the adhesive material, gg 328 i3 the particle size ranging from 0.01 pm to a few mi- 9 2565 1985 56 crometers, the particles being of a substantially 10 2275 65 34 unlform s1ze, the cross-sect1ona1 contours of the l; 88 233 gg particles being evenly curved, and 13 770 710 56 the ratio of the maximum and the minimum dimeni 238 Egg '3? sion of the particles being less than 10 to l. 16 2875 2365 96 2. A glue mixture as claimed in claim 1 wherein said 17 2950 2%6; particles are selected from the group consisting of me :3 32 3 19 tallic oxides and silicon oxide. 20 840 700 31 V 3. A glue mixture as claimed in claim 1, wherein the g; 238 3Z8 2% poorly conducting substance is an electrically insulat- 23 560 500 240 mg substance. 3; 23 8%? 4. A glue mlxture as cla1med in claim 1, wherein the 26 1314 84 903 poorly conducting substance is a semiconductive sub- 27 2380 1815 78 stance.

Table 2 o M 0 s10 SiC Mica Particle pm Particle pm Particle p.111 Particle #m Particle 14m size size size size size Q1 0.3 14.1 0.1 16.0 4.0 12.5 1 64.0 0.2 4.4 0.6 24.2 0.2 16.0 5.0 12.5 2 22.4 0.3 8.9 1.0 35.9 1 0.3 4.0 6.0 25.0 3 7.6 0.4 8.9 2.0 14.8 0.5 20.0 7.0 12.5 4 2.0 0.5 11.1 3.0 6.2 0.6 8.0 8.0 25.0 5 2.0 0.6 11.1 4.0 2.4 0.7 4.0 9.0 12.5 6 0.7 0.7 15.7 5.0 2 .4 0.9 4.0 10.0 8 1.0 0.8 8.9 1.0 8.0 l3 0.3 0.9 1.1 8.0 1.0 11.1 1.2 4.0 H 4.4 L3 8.0 1.2 4.4 1.3 6.7 1.4 1.5 4.4 1.6 1.7 1.8 1.9 2.0 Aver. 0.74 Aver. 1.24 Aver. 0.59 Aver. 6.62 Aver. 1.66

particles are substantially globular. 8. A glue mixture as claimed in claim 1, wherein the pamcle to 3 particles are of zinc dioxide.

6. A glue mixture as claimed in claim 1, wherein the A glue "fixture as Clalmed clalm Where! the particles are of aluminum oxide. f l

5. A glue mixture as claimed in claim 1, wherein said particle content is lOO to l,500 per cent by weight 0 the adhesive material.

7. A glue mixture as claimed in claim 1, wherein the 

1. A GLUE MIXTURE TO BE USED IN OBJECTS SUBJECT TO STRONG ELECTRICAL FIELDS SUCH AS IN GLUE SPLICES OF ELECTRICAL CAPACITOR TISSUES, COMPRISING: A. A GLUE COMPONENT SELECTED FROM THE GROUP CONSISTING OF METHYL CELLULOSE, STARCH, ARABIUM, DEXTRIN AND PALSTIC ADHESIVE; AND B. PARTICLES OF A POORLY CONDUCTING SUBSTANCE MIXED WITH THE ADHESIVE COMPONENT, THE PARTICLE CONTENT RANGING FROM 50 TO 5,000 PER CENT BY WEIGHT OF THE ADHESIVE MATERIAL, THE PARTICLE SIZE RANGING FROM 0.01 UM TO A FEW MICROMETERS, THE PARTICLES BEING OF A SUBSTANTIALLY UNIFORM SIZE, THE CROSS-SECTIONAL CONTOURS OF THE PARTICLES BEING EVENLY CURVED, AND THE RATIO OF THE MAXIMUM AND THE MINIMUM DIMENSION OF THE PARTICLES BEING LESS THAN 10 TO
 1. 2. A glue mixture as claimed in claim 1 wherein said particles are selected from the group consisting of metallic oxides and silicon oxide.
 3. A glue mixture as claimed in claim 1, wherein the poorly conducting substance is an electrically insulating substance.
 4. A glue mixture as claimed in claim 1, wherein the poorly conducting substance is a semiconductive substance.
 5. A glue mixture as claimed in claim 1, wherein said particle size is 0.2 to 3 Mu m.
 6. A glue mixture as claimed in claim 1, wherein the particle content is 100 to 1,500 per cent by weight of the adhesive material.
 7. A glue mixture as claimed in claim 1, wherein the particles are substantially globular.
 8. A glue mixture as claimed in claim 1, wherein the particles are of zinc dioxide.
 9. A glue mixture as claimed in claim 1, wherein the particles are of aluminum oxide. 